Cytoskeletal remodeling during myotube assembly and guidance: coordinating the actin and microtubule networks

Abstract

The formation of a multinucleated muscle fiber from individual myoblasts is a complex morphological event that requires dramatic cytoskeletal rearrangements. This multistep process includes myoblast fusion, myotube migration and elongation, myotube target recognition, and finally attachment to form a stable adhesion complex. Many of the studies directed towards understanding the developmental process of muscle morphogenesis at the cellular level have relied on forward genetic screens in model systems such as Drosophila melanogaster for mutations affecting individual stages in myogenesis. Through the analyses of these gene products, proteins that regulate the actin or microtubule cytoskeleton have emerged as important players in each of these steps. We recently demonstrated that RacGAP50C, an essential protein that functions as a cytoskeletal regulator during cell division, also plays an important role in organizing the polarized microtubule network in the elongating myotube. Here we review the current literature regarding Drosophila myogenesis and illustrate several steps of muscle development with respect to the diverse roles that the cytoskeleton plays during this process. Furthermore, we discuss the significance of cytoskeletal coordination during these multiple steps.

Keywords: actin; cytoskeleton; microtubule; muscle; myotube guidance.

Figure 1

Multiple roles of the cytoskeleton during Drosophila myogenesis. (A) Muscle assembly requires multiple steps, some of which occur simultaneously. Shown is a myoblast fusing to a multinucleated myotube as it elongates and searches for the proper attachment sites. The actin cytoskeleton (shown in red) is required for fusion at the central region of the myotube as well as at the myotube ends for target recognition during muscle attachment site selection. A polarized microtubule array (green) is found along the linear axis of the myotube with minus ends anchored near the nuclei, at the interior of the muscle fiber (black) and the plus ends directed outwards. (B) Myoblast-Myotube Fusion. Recent studies have shown that a dense actin focus (red oval) is critical for myoblast fusion to a multinucleated myotube. The formation of the actin focus is driven by actin regulators such as WASp, WIP and Arp2/3. (C) Myotube Elongation. Myotube elongation requires a uniform microtubule array based at the nuclear periphery. Through interaction with Pavarotti, RacGAP localizes to the nuclear periphery where it colocalizes with the microtubule nucleator protein γ-tubulin to establish a polarized microtubule array. It is not clear whether RacGAP and γ-tubulin directly interact in the myotube. (D) Target Recognition. Many molecules accumulate at myotube ends where they are required for the selection of appropriate attachment sites. These molecules include the well-known axon guidance molecules Slit and Roundabout. Kon-tiki and Echinoid are also involved in myotube target recognition, and both molecules are able to bind Grip, a downstream scaffolding protein. These molecules are required for target recognition within the same subset of myotubes, but it is unclear how they are coordinated. The Drosophila homologue of GIT was also recently implicated in target recognition. To date, common downstream signals that direct cytoskeletal rearrangements required for target recognition remain largely unknown. MSP-300 may be one molecule that mediates actin dynamics at the plasma membrane.